ABSTRACT Fibromyalgia syndrome (FMS) is characterized by chronic widespread pain, affect/sleep disturbance, fatigue, and cognitive dysfunction. People with FMS report high distress, disability, lost productivity, and poor quality of life. FMS is estimated to affect over 6.5 million Americans and thus represents an important public health problem. A range of pharmacological agents have been tested (antidepressants, antiepileptic medications), but findings indicate only small effects with a high rate of dropout due to adverse side-effects. Cognitive behavioral therapy (CBT) and exercise have also been tested, but have only small to modest effects. Thus, there is an urgent need to develop readily available, affordable, safe, and practical adjunctive approaches to manage FMS symptoms. In this R21 Exploratory/Developmental research grant, we will examine the extent to which a morning light treatment improves function, pain intensity, and pain sensitivity in people with FMS (n=60). We will assess baseline function, pain intensity, and pain sensitivity. Subjects will then be randomized (stratified 18-29 years vs. older): to one of two treatments: 4 weeks of bright morning light (1 h/day) or 4 weeks of dim (placebo) morning light (1 h/day). Subjects will have reassessments of function, pain intensity, and pain sensitivity at mid-treatment and post-treatment. Our pilot data suggests that morning bright light treatment can meaningfully reduce FMS symptoms. In the proposed study, we will reduce subject burden and increase innovation by testing a wearable light device (bright vs. dim Re-timer) with objective measures of treatment compliance. Aim 1 is to determine the effect of bright versus dim (placebo) morning light treatment on function, pain intensity, and pain sensitivity in individuals with FMS. We hypothesize that function, pain intensity, and pain sensitivity will improve significantly more in the bright light treatment group than in the dim (placebo) light treatment group. Aim 2 is to determine the effect of bright vs. dim (placebo) morning light treatment on potential treatment mediators: morningness-eveningness (a proxy measure of circadian timing), sleep, negative affect, pain catastrophizing, and analgesic medication use. We hypothesize that these variables will significantly improve more with bright light than dim light treatment, and changes in these potential mediators will be associated with changes in function, pain intensity, and pain sensitivity. The Exploratory Aim is to explore whether obstructive sleep apnea severity and/or subject sex moderate treatment effects. The results will provide a foundation for a well-powered R01 RCT to test if morning light (bright vs. placebo), as a non- pharmacological adjunctive treatment, can enhance existing treatments for FMS (e.g. CBT, internet-based CBT, exercise) to improve symptom management. This R21 grant application answers PA-16-008, which calls for ?interdisciplinary research to decrease symptom burden in persons with chronic illness through developing and testing innovative, cost-effective, targeted interventions? and also addresses NINR's strategic plan calling for ?better symptom management of chronic pain and innovative behavioral treatments applied at home?.